Shutters for windows, doors and building openings are now commonly made with rectangular frames of extruded thermoplastic material. Louvres extend across the frame which are also usually made of thermoplastic material.
The shutter frame is itself hingedly mounted by suitably hinges in a border frame, and the border frame is secured to the building fabric around the opening.
The border frame is also formed on thermoplastic material.
The shutter may thus be swung open, or swung closed, as desired.
In the past it has been usual to provide some form of friction lock device on the shutter frame, and the border frame which locks the shutter frame closed.
These devices typically have been similar to cupboard door locks, such as some form of spring loaded ball device, in one frame, and a socket in the other frame. The ball seats in the socket when the frame or door is closed, and holds it closed.
Such devices have been satisfactory, as locks, in most cases. However over time they are prone to failure, due simply to wear, unless made to high standards.
A more serious disadvantage is that they are troublesome to install. The installer must use considerable skill, and must get the ball, and socket in the correct positions, and in alignment, so as to work effectively. This is particularly troublesome when working with frames made of thermoplastic material. Such frames are usually formed by extrusion, and define a series of hollow tubes, and thin walls enclosing the tubes.
Securing a typical ball and socket type lock, or indeed many similar types of locking devices, in frames with hollow interiors, requires skill and ingenuity.
A still further problem is that such locks in the past have been usually installed only along the bottom portions of the two frames.
The ball (or other lock) exerts continuous pressure, tending to widen the gap between the border frame and the shutter frame. This stresses the frame itself. It also stresses the hinges.
Eventually, especially in the case of frames made of thermoplastic material, the frames become distorted. Thermoplastic material is well known to have a fatigue life much less than that of wood, or metal. Repair of a distorted shutter frame is difficult especially since such thermoplastic material once damaged will always be weak.
Clearly there is a need for a lock for such thermoplastic shutter frames, which is of low cost, and which can be incorporated in the frames themselves during original manufacture, thus avoiding later installation, and especially which avoids the problem of stressing the frames and distorting them.